Patchett et al. in European Patent Application No. 12,401 disclose that carboxyalkyl depeptide derivatives of the formula ##STR2## are useful antihypertensive agents due to their ability to inhibit the angiotensin converting enzyme. Among the compounds disclosed by Patchett et al. are those wherein R.sub.4 and R.sub.5 join together to form an alkylene bridge of from 2 to 3 carbons and one sulfur atom, an alkylene bridge of from 3 to 4 carbons containing a double bond or an alkylene bridge as above substituted with a hydroxy, lower alkoxy, lower alkyl, or di(lower alkyl) group.
Mercaptoacyl derivatives of proline and substituted prolines are known to be useful hypotensive agents due to their angiotensin converting enzyme inhibition activity. Ondetti et al. in U.S. Pat. No. 4,105,776 disclose such compounds wherein the proline ring is unsubstituted or substituted by an alkyl or hydroxy group. Ondetti et al. in U.S. Pat. No. 4,154,935 disclose such compounds wherein the proline ring is substituted with one or more halogens. Ondetti et al. in U.K. Patent Application No. 2,028,327 disclose such compounds wherein the proline ring is substituted by various ethers and thioethers. Krapcho is U.S. Pat. No. 4,217,359 disclose such compounds wherein the proline ring has a carbamoyloxy substituent. Krapcho in U.K. Patent Application No. 2,039,478 and U.S. Ser. No. 99,164 filed Nov. 30, 1979, now U.S. Pat. No. 4,311,697, discloses such compounds wherein the proline ring has a diether, dithioether, ketal or thioketal substituent in the 4-position. Krapcho in U.S. Ser. No. 164,985 filed July 1, 1980, now U.S. Pat. No. 4,316,905, discloses such compounds wherein the proline ring has a cycloalkyl, phenyl, or phenyl-lower alkylene substituent. Ondetti et al. in U.S. Ser. No. 51,772 filed June 25, 1979, now U.S. Pat. No. 4,234,489, disclose such compounds wherein the proline has a keto substituent in the 5-position. Krapcho et al. in U.S. Ser. No. 162,341 filed June 23, 1980, now U.S. Pat. No. 4,310,461 disclose such compounds wherein the proline has an imido, amido, or amino substituent in the 4-position. Iwao et al. in U.K. Patent Application No. 2,027,025 disclose such compounds wherein the proline has a phenyl or a 2- or 4-hydroxyphenyl substituent in the 5-position.